The long-range goal of this project is to understand how descending neural systems modify information processing in the cochlea and cochlear nucleus. Existence of descending innervation to hair-cell systems in most vertebrate species examined is strong presumptive evidence for their importance. Descending neural systems have high relevance for human health because they function to: 1) adjust the best operating range of the inner ear, 2) reduce the effects of masking noise, and 3) protect the inner ear from overstimulation by intense sound. Interruption of these systems would severely damage important processes like speech discrimination and render the inner ear more vulnerable to damage. This project will concentrate on two particular descending systems, the medial olivocochlear (MOC) neurons and the middle ear muscle (MEM) motoneurons. Both types of these neurons respond to sound and form reflexes that send information to the auditory periphery. The intermediate steps of these reflexes, which pass through the cochlear nucleus of the brainstem, are not understood. The process by which the brain modulates these reflexes is also not understood. We will study the reflexive and modulatory neural pathways using light and electron microscopy, recordings, and lesions. The experimental model will be rodents. These experiments will answer questions like, Which neurons are the important reflex interneurons for the MOC and MEM reflexes? and What types of synapses do the interneurons make on MOC neurons and MEM motoneurons? We will also answer questions such as, How is characteristic frequency mapped by MOC neurons in the brain? and How do MOC neurons respond in the awake animal? The knowledge obtained will enable us to precisely specify the entire pathway of both of these reflexes. It will also greatly increase our understanding of both the descending and the ascending auditory pathways. This project aims to understand the neural pathways of two brainstem reflexes of the auditory pathway: the medial olivocochlear reflex and the middle ear muscle reflex. Both of these reflexes have high relevance for public health because they protect the ear from damage and reduce the effects of noise masking. An understanding of the neural pathways is important to understand how these reflexes accomplish their functions.